Vertebroplasty and kyphoplasty affect vertebral motion segment stiffness and stress distributions - A microstructural finite-element study

Study Design. The mechanical behavior of a thoracic motion segment following cement augmentation was studied using the finite-element method. Objective. To examine effects of cement augmentation on motion segment stiffness and load transfer. Summary of Background Data. Vertebroplasty and kyphoplasty procedures are meant to stiffen and strengthen the vertebral body, but the optimal cement volume and placement to achieve these goals without altering load transfer to adjacent segments are unknown. Methods. A microstructural finite-element model of a vertebral motion segment was constructed from micro-CT images. Microdamage within the vertebral body trabecular structure was modeled using an elasto-plastic modulus reduction scheme. Three motion segment damage models were created: I = 18% apparent modulus reduction (least damage), II = 45%, and III = 85% (most damage); and several one- and two-segment polymethylmethacrylate cement repair strategies (partial fill kyphoplasty, replacement of bone and marrow; and both partial fill and complete fill vertebroplasty, replacement of marrow only) were studied. Average disc and bone stresses and motion segment apparent compressive stiffness were compared with baseline (undamaged and untreated) simulation results. Results. In terms of maximizing stiffness and minimizing stress alterations in the adjacent vertebral body and increasing motion segment apparent stiffness, we found that, other than complete fill, the most effective single-segment cement repair strategy was vertebroplasty on the periphery of the superior segment overlying the disc anulus (< 0.1% overall vertebral body bone stress alteration and 83% stiffness increase, respectively, damage Model III). Two-segment vertebroplasty (all repair models) restored motion segment stiffness to baseline levels in all damage models, while single-segment vertebroplasty (all repair models) restored stiffness to baseline levels only in damage Model I. Single- and two-segment kyphoplasty was effective in restoring stiffness to baseline levels for Model I only. Compared with the baseline model, cement augmentation decreased average treated segment bone stresses (up to 66%, complete fill vertebroplasty elasto-plastic modulus reduction Model III), increased average intervertebral disc nucleus stresses (up to 59%, kyphoplasty elasto-plastic modulus reduction Model III), and increased average adjacent segment, endplate region stresses (up to 2.8%, kyphoplasty elastoplastic modulus reduction Model II). Adjacent (untreated) segment peak bone stresses were increased (up to 45%, kyphoplasty, Model III) in endplate regions underlying the intervertebral disc nucleus. Conclusions. The damage-repair simulations indicated that cement augmentation improves motion segment stiffness but substantially alters bone stress distributions in treated and adjacent segments.